State of the art pneumatic rubber tires are typically prepared with a rubber tread having a cap/base construction. In such construction the "cap" refers to the outer portion of the tread having a pattern of grooves (or "valleys") and lugs ("raised portions defining the valleys") which together impart desirable characteristics to a rolling tire as the lugs contact the ground. A primary purpose for dividing a tread into "cap" and "base" portions is to reduce the tire's rolling resistance relative to that of a tire having the same tread except that it is prepared from a single rubber compound, and to do so without unduly sacrificing its traction (skid resistance) or tread wear.
The cap preferably extends inwardly to just beneath the bottoms of the grooves. Rubber for the cap ("cap compound") is compounded to provide good skid resistance, tread wear and rolling resistance.
The base lies beneath and contiguous to the cap, thus being sandwiched between the cap and outermost belt of the tire's carcass. Rubber for the base ("base compound") is compounded to enhance rolling resistance and durability of the tire. Such constructions and different rubber compounds in the formulation of the cap and base, are disclosed in U.S. Pat. No. 3,157,218 to Brown; and U.S. Pat. Nos. 5,174,838 and 5,284,195 to Sandstrom et al.
In the art of tire construction, it is deemed that increasing the base's thickness (gauge) provides an improvement in rolling resistance (lowered resistance to rolling the tire, usually under loaded conditions) relative to a thinner base formed of the same compound. However, increasing the thickness of the base tends to produce "peaking" of the base during the molding and curing of the tire. "Peaking" refers to protrusion of base compound into the lower portion of the cap, including its lugs, which cap is formed from a cap compound different from that used to form the base. Peaking is conducive to undue wear of the lugs and the development and propagation of cracks in the cap. In extreme cases of peaking, when the lugs are worn so far as to expose the base rubber compound, the original traction and tread wear of the cap are severely compromised.
Peaking is attributed mainly to the viscosity of uncured base compound being substantially lower than that of the cap compound. Therefore, during curing of the tire at elevated temperature and under pressure, because the base compound tends to flow more easily than the cap compound, base compound seeks to protrude into the lugs and grooves as the cap is formed in a mold.
To minimize unacceptable protrusion (and peaking) it is known that it is desirable to increase the green strength and viscosity of the uncured base compound while maintaining a satisfactory resilience (rebound value) of the cured base compound. In the art, the viscosity of uncured base rubber compound has been increased with conventional compounding ingredients, such as, for example, carbon black content; and, optionally, reducing the oil content of the base compound. However, it is also conceded in the art that such expedients for increasing viscosity tend to increase the hysteresis of the cured base compound resulting in poorer rolling resistance, and otherwise compromise the desirable properties inculcated by a choice base compound. In the '838 patent and other prior art treads of similar construction, the Mooney viscosity of the uncured base rubber compound of choice is lower than that of the uncured cap compound.
The '838 patent specifically teaches the use of "high" trans 1,4-polybutadiene (so-termed because its "trans" content is at least 70%) for a base compound; and the use of trans 1,4-polybutadiene has been disclosed for various purposes, including tire tread rubber compounds and increasing green strength of rubber mixtures in Japanese Patent Publications Nos. 60-133,036: 62-101,504 and 61-143,453 and U.S. Pat. No. 4,510,291. All failed to realize that improving green strength was a good criterion for choice of a base compound only if the green strength was maintained in the higher portion of the range of normal processing temperatures, specifically at a temperature of at least 100.degree. C. Further, the prior art failed to disclose that the critical criteria for desirable green strength was the stress/strain values of the uncured compound at 100.degree. C., as for example, measured by the Monsanto RPA Test, which permits processing of the ingredients at a temperature in the range from about 100.degree. C. to about 190.degree. C.
In the '838 and '195 patents a desirable difference in flow properties of the cap and base compounds was obtained by using specified amounts of trans 1,4-polybutadiene or trans 1,4-polyisoprene. Each is substantially crystalline, and curing the tire requires conversion of each into a satisfactory rubber which is homogeneously distributed within the cured base compound. In this invention, partially crosslinked NR having minimal crystallinity is substituted for the crystalline rubbers previously used, thus avoiding the mechanism for homologating the latter. Further, crystalline rubbers are more sensitive to elevated temperatures such as are generated in a non-productive stage of compounding, resulting in less flexibility and less latitude for processing discrepancies.
The base rubber compound used to build a tire of this invention may also be used to compound a `ply coat` and a `wire coat`. Conventional compounds for the ply coat and the wire coat are predominantly NR and are, therefore, prone to flow excessively during cure, resulting in displacement of the wires in the belts, or in the cords of fabric, or in distortion of the circumferential profile of the inner liner compound. Typical recipes for a wire coat and a ply coat are found in The Vanderbilt Rubber Handbook, Thirteenth Edition pg 606, Table 7 (published by R. T. Vanderbilt Company Inc. 1990). It is highly desirable that the flow of the wire coat and ply coat compounds be more controlled and limited than it is with conventionally used NR blended with less than 30 phr (parts per hundred parts rubber by weight) cis 1,4-polybutadiene, and that such controlled flow be obtained without adversely affecting the desirable properties of the wire coat and ply coat.
Partially crosslinked natural rubbers are conventionally used as processing aids for improving the extrusion, calendering and general processing properties of natural and synthetic rubbers. In particular, partially crosslinked natural rubbers have been used in the formulation of compounds for rubber hose and rubber conveyor belts. There has been no suggestion that any of these rubbers be used to modify the viscosity of a cap or base compound specifically to affect the Mooney (ML1+4) value at 100.degree. C., and no reason to expect that such change in value might provide the basis for controlling peaking during the cure of a tread having cap/base construction, without adversely affecting the desirable physical properties of the cured tire.
Despite the relatively low glass transition temperature Tg of the specified partially crosslinked NR which Tg might be expected to lower the Mooney viscosity of the base compound at elevated processing temperatures, the (ML1+4) viscosity at 100.degree. C. of the novel base compound remains in the range from 50 to 60, thus allowing a processing temperature in the range from about 100.degree. C. to 180.degree. C. This range of processing temperature, in at least one non-productive and one productive stage, allows optimum compounding of the base compound though this range is higher than that generally deemed desirable to process prior art base compounds.